namespace and folders

[Gregory Sloop]

Using mbox format
and this namespace

namespace {
  inbox = yes
  hidden = yes
  prefix = INBOX/
}

[Though I've also tried, seemingly, a million different namespace options to "fix" my problem.]
It's a migrated setup from a 1.x dovecot install from Ubuntu 12.04, now on 16.04

The problem is in TBird [I haven't tested with any other mail clients] and IMAP; with that name-space I get all the folders both at the root level [same as inbox], as well as below inbox.
[The same folders, pointing at the same underlying messages.]

For example, if there's a folder "blah" and I subscribe to it, "blah" shows both at the same level as inbox, as well as a sub-item of inbox.
If I unsubscribe from folder "blah" it also unsubscribes from the subfolder "blah" under inbox, and at the root level.
[I hope that's clear enough...]

This is not a terrible problem, but it's cosmetically ugly and confusing for users.
I'd prefer to only see these folders as subscribed as sub-folders [or sub-items] under inbox.

I'd guess I'm making some stupid beginner error, but I've not been able to figure it out, as of yet. Any help would be greatly appreciated.

-Greg